Automatic reverse current switch



NOV. 14, 1939. J LOY 2,179,794

AUTOMATIC REVERSE CURRENT SWITCH Filed D90. 1, 1957 Q N W r\ :u

WITNESSES; INVENTOR- g James Mim A TTORN E YS.

W A Q W Patented Nov. 14, 1939 UNITED STATES PATENT OFFICE AUTOMATICREVERSE CURRENT SWITCH Application December 1, 1937, Serial No. 177,458

1 Claim.

This invention relates to automatic reverse current protection, andparticularly to automatic switch mechanism for that purpose. I aim tomake the mechanism reliable, even when traversed by stray alternatingcurrents of appreciable magnitude, and free of any liability for aswitch to seal in or stick in case of too rapid reversal of current; toprovide ample margin of operating power to insure reliable action; andto obviate high cost of maintenance, which has been a general drawbackof reverse current switch mechanisms heretofore tried. Another advantageafforded by the invention is that standard devices, already commerciallyavailable, can be used for the component parts of my switch mechanism orsystem, so that it need not be unduly expen sive. The invention isespecially adapted and advantageous for the protection of lead sheathedcable and the like against destructive electrolysis due to straycurrents, and it is explained hereinafter with special reference to thisuse. Other underground structures may, of course, be similarlyprotected,such as Water or other pipes, structural steel members, etc.

A great deal of expensive underground lead sheathed electric cable isyearly rendered useless (and has to be removed and replaced, at largeexpense) owing to impairment of the lead sheath by electrolysis, due tostray currents. This is especially so in places where direct currentrailways operate. Merely to (electrically) bond or connect the leadsheath solidly to a substation negative bus bar or other drainage pointdoes not solve the problem; for under the varying conditions of railwayoperation, the lead sheaths of cables may become negative in relation tothe negative substation bus or other drainage point, instead ofpositive. Hence an automatic switch is needed in the bond to prevent thecurrent from reversing to the cable sheaths. Protective devices recentlyemployed for cables of telephone systems are not well suited for powercable, in many cases.

These problems and the drawbacks of automatic reverse-current switcheshitherto used (whose disadvantages, such as suggested above, are largelydue to their being self-actuated) are met and overcome by my invention.Still other features and advantages of the invention will appear fromthe following description of species or forms of embodiment, and fromthe drawing.

The drawing is a schematic diagram of operating devices and connectionssuitable for the purposes of the invention.

The drawing shows a portion of the undergroimd metallic structure 5 tobe protected,-such as a cable sheath (or system of such sheaths), apiping system, etc.,and a drainage point 6 (such as the negative bus-barof a direct current railway substation, or the like), with a bond .orcircuit 1 interconnecting them. This circuit 1 may include a fuse 8 forprotecting the switch mechanism against extraordinary conditions, aswell as an interrupting switch It that is intended to open in case thecurrent flow through the circuit 1 reverses. The circuit 1 may alsoinclude a shunt portion H of definite low resistance, besides otherresistance l2. The switch Ill may be of a self-opening type, with anoperating coil I3 that closes it when energized by current flowtherethrough. Such current may be supplied from an auxiliary power (andcontrol) circuit l4 including said coil l3 (and also shown as includingprotective overload fuses 5, l5), under the control of an auxiliaryswitch relay I6 whose coacting movable and fixed contacts l8 areconnected in (or across) said circuit I4.

As shown, the auxiliary switch relay It includes additional coactingmovable and fixed contacts 20, 2|, the former of which is connected toan extension of one side of the circuit M. The operating coil 22 of thisrelay I6 is in a circuit 24 one end of which is connected to one side ofcircuit l4, and may include a resistance 25. The other end of circuit 24is connected to the movable (floating) contact 26 of a sensitivepolarized potential relay switch 21, for controlling the energization ofcoil 22 of relay Hi. The circuit 24 has a branch extension 28 connectedto the contact 2| of relay switch IS. The relay switch 21 is shown as ofknown type having a moving coil 30 operating between the poles of a(soft iron) core 30a in the heavily saturated field of a coil 3|, whichlatter is connected by a circuit 3|a, to any suitable source ofenergy,as here shown, across the auxiliary power circuit |4,-so as toserve as a polarizing coil. Thus the circuit for energizing coil 3| isfrom one side of circuit through one lead Sla, through the coil 3|itself and through the other lead 3| a to the other side of circuit M.The saturation of the magnet poles of core 30a makes the relay 2'!insensitive in its operation to any ordinary voltage change in circuitI4; and the mounting of the moving coil 30 provides appreciable dampingof the movements of the relay and its floating contact 26,-which reducesundesirable alternating current effects without interfering with thedesired operation of the relay. The movable contact 26 of the relay 21coacts alternatively with front and back confill tacts 32, 33, accordingto the direction of current flow through the coil 30. To contact 32 isconnected an extension of the same side of circuit I 4 to which themovable contacts I1, 20 of relay switch It are connected, and to contact33 is connected a short-circuit lead 34 that is connected to that partof circuit 24 which connects resistance 25 to coil 22. The resistance 25allows coil 22 to be short-circuited through the connection 34, and thusde-energized, without opening control circuit M or interfering withproper operation of the system.

For controlling the energization of coil 22 of relay 16 according to thepotential from time to time existing between the sheath of cable 5 andthe drainage point 5, the coil 30 of relay switch 2'! is connectedbetween them as a potential coil, in parallel with the interruptingswitch or circuit breaker ill, being included in a circuit 3'! which mayconveniently be connected to said circuit 1 in shunt with switch l andalso (preferably) with the portion H and the resistance 12. This shuntcircuit 31 may itself include resistance 33 and a fuse 39, to affordprotection against excessive current due to high voltage drop betweenthe contacts of switch l0 when open. Thus the circuit for initiallyenergizing coil 30 is from one end of resistance H in circuit 1 throughlead 3?, coil 39, and lead 37 (including resistance 38 and fuse 39) tocircuit 1 again. An adjustable resistance ill may be connected betweenthe circuits 1 and 37, as shown, to assure satisfactory relative valuesof the current through coil 3! when switch it is open and closed. Thisconnection may not materially affect the current through coil 30 whenswitch H! is closed; but when switch ill is open, resistances II and 48are in parallel with coil 30.

The relay switch 2? is shown in neutral position, with its contact 26disengaged from both contacts 32, 33, and with relay switch It and maincircuit-breaker ll] both open,-a condition naturally obtaining when thesystem is initially being put into service, before any of its circuitsare energized. Ordinarily, the sheath of cable 5 should be at a positivepotential relative to the drainage point 6. When this positive potentialreaches a value of about .25 volt (or whatever suitable value isdetermined by the design and setting of relay 2i and the value ofresistance 33), contact 26 engages contact 32 and thus completes thecircuit through coil 22, energizing the latter to operate relay l6 andclose contacts ll, 58. The circuit which thus initially energizes coil22 is from one side of circuit M through contact 32, contact 26, onelead 24, coil 22, and the other lead 24 and its resistance 25 to theother side of circuit [4. This closes contact ll to complete circuit Mand energize coil iii of switch ill, closing the latter so as tocomplete circuit l, and thus providing a low resistance path for currentfrom cable sheathing 5 to drainage point t. The closing of switch If]also shunts out resistance 38 in series with coil 32, leaving coil 38subject only to the small voltage drop (measurable in millivolts)determined by the relative values of resistances H and 6E]. The closingoperation of relay It as above set forth also closes contacts 20, 2|,completing a circuit M-28-2422-24-2525- M which energizes coil 22independently of engagement between contacts 23 and 32.

If the voltage between cable sheath 5 and drainage point 6 shoulddecrease to zero or become reversed, so that the current through circuitI correspondingly decreases or is reversed, then the current throughcoil 38 of relay 21 will recrease or reverse in proportion. As thecurrent through this coil 39 decreases sufficiently, the relay 2'! willoperate to open the circuit through its contacts 26 and 32,which,however, are still shunted by the contacts 20, 2E of relay H5. By thetime the current flow through coil 39 has decreased to zero, or hasreversed slightly (as determined by the setting of the relay 27!), therelay 2'! will have closed its contacts 26, 33, shunting coil 22 ofrelay l5 through circuit 34. As this coil 22 of relay i6 is in circuitthrough contacts 20, 2! independently of contacts 26, 32, coil 22 is notdeenergized (nor relay IB operated) by mere separation of contacts 26and 32, but only by engagement of contacts 26 and 33. The shunt circuitthus established extends from one side of circuit i l through contacts20 and 2!, circuit 2M one lead 2 1, contacts 28 and 33, circuit 3%, andthe other lead 24 (including resistance 25) to the other side of circuitM. This shunts out coil 22, deenergizing it and allowing contacts ll,2%? of relay l6 to drop out and open circuit M (as well as circuit 28),which in turn deenerglzes coil 33 and allows switch ill to open. Thisbreaks the circuit 7 and prevents injury to the cable sheathing bycurrent flow in the wrong direction,--resistance 38 being so high thatno appreciable flow to cause electrolysis can take place through circuit31.

The proper value for resistance 38 and for the sensitivity of relay 2'!are determined by the maximum reverse voltage likely to be experienced,and from the closing and opening voltages desired for the switch id.Types of switches and relays available commercially give performancewell within what is needed under practical conditions. For the relay 2?,a type that is appreciably damped against the effect of strayalternating current is naturally preferred; and since its contacts arerelieved of load as soon as contact is made, no objectionable stray aterposed in said connection, and an operating coil for causing saidinterrupter to open and close, of a polarized potential relay switchhaving a potential winding connected in parallel with said interrupter,across its gap, and a polarizing winding with connections for energizingit, an auxiliary power and control circuit independent of said drainageconnection, with a relay switch controlling said latter circuit and theenergization of the operating coil of said inter-' rupter, and circuitmeans for the energization and operation of said latter relay switchcontrolled by said polarized potential relay switch.

JAMES M. LOY.

